1. Field of the Invention
The present invention relates to an image reader and a digital copier, more specifically, an image reader capable of reading images at an arbitrary scanning resolution and a digital copier capable of copying at an arbitrary zoom ratio.
2. Description of the Prior Art
High-speed and multifunctional digital copiers have been available recently, and fast speed for first copy time is demanded in the processing of contraction or enlargement.
Hereinafter, an example of a conventional digital copier will be described with reference to FIGS. 9 and 10. FIG. 9 is a block diagram showing the flow of image signals of a conventional digital copier. An image sensor 71 scans a document and converts reflected light from the document into an electric signal. An A/D converter 72 converts an analog image signal from the image sensor 71 to a digital image signal. An image processing circuit 73 performs image processing such as edge enhancement, trimming, and half-tone processing or editing processing with respect to the digital image signal. A line buffer memory 74 buffers the image signal from the image processing circuit 73 for speed adjustment for outputting the image signal to a laser driver 78. The laser driver 78 drives a semiconductor laser 79 to output laser beams for formation of electrostatic latent images.
FIG. 10 shows a schematic structure of a conventional digital copier. The digital copier includes an image scanning part A and a laser printing part B. The image scanning part A includes an ADF (auto document feeder), a document table 0, a constant speed unit 3 including an exposure lamp 1 and a first mirror 2, a half speed unit 6 including a second mirror 4 and a third mirror 5, a lens 7, and an image sensor 8.
The laser printing part B includes a laser scanner unit including a semiconductor laser, a polygon motor, a polygon mirror and a laser optical system, a mirror 41, a photoconductor drum 9, a main charger 10, a developer 11, a transfer charger 20, a cleaner 35, an electricity removal lamp 36, a conveyer belt 30, a fixer 31, a guide 32, a paper ejection roller 33, paper feed rollers 12, 13, and 14, a guide 18, a timing roller 19 and a sorter 34.
The operation of the conventional digital copier having the described structure will be described below. Aplurality of documents placed on the ADF 50 are fed one by one by the ADF and placed face down on the document table 1 formed of a transparent glass sheet. The document is exposed to light by the exposure lamp 1. The first mirror 2 reflects the reflected light from the document in the direction of the second mirror 4. The constant speed unit 3 including the exposure lamp 1 and the first mirror 2 scans the document while moving at a constant speed in the direction shown by arrow P. The second mirror 4 and the third mirror 5 reflect the reflected light from the first mirror 1 further. The half speed unit 6 including the second mirror 4 and the third mirror 5 moves at a speed half that of the constant speed unit 3 in the same direction as the constant speed unit 3. The reflected light from the document is converged to the image sensor 8 by the lens 7.
The image sensor 8 is a linear sensor that is longer in the direction perpendicular to the direction of the movement on the surface of the document. In the following description, an electric scanning direction along the longitudinal direction of the image sensor 8 is referred to as a primary scanning direction, and a mechanical scanning direction along the direction of the movement of the constant speed unit 3 is referred to as a secondary scanning direction.
The photoconductor drum 9 rotates at a constant speed in the direction shown by arrow R. The main charger 10 charges the photoconductor drum 9 uniformly. The laser beams radiated from the laser scanner unit 40 is reflected at the mirror 41 for exposure and scanning on the photoconductor drum 9 so that electrostatic latent images are formed on the photoconductor drum 9. The developer 11 develops the electrostatic latent images with toner, and forms toner images on the photoconductor drum 9. In the photoconductor drum 9, a direction parallel to the rotation axis corresponds to the primary scanning direction, and the rotation direction of the photoconductor drum corresponds to the secondary scanning direction.
Recording paper cassettes 15, 16, and 17 hold a plurality of recording papers. The recording paper cassettes each hold different types of recording papers. The recording paper cassettes are removable so that the cassettes holding recording papers having necessary size and direction are removably mounted to the copier. The paper feed rollers 12, 13 and 14 feed one recording paper at a time from the recording paper cassettes. The guide 18 guides the fed recording paper to the ting roller 19. The timing roller 19 adjusts feed timing and performs registration so that the toner images on the photoconductor drum 9 are on the fed recording paper.
The toner images on the photoconductor drum 9 are transferred onto the recording paper by an electric field generated by the transfer charger 20. The conveyer belt 30 moves in the direction shown by arrow Q and conveys the recording paper with the toner images transferred to the fixer 31. The fixer 31 fixes the toner images onto the recording paper by heat. The recording paper with the images fixed thereon is guided by the guide 32 to the paper ejection roller 33. The paper ejection roller 33 ejects the paper to the sorter 34. The sorter 34 is provided with a plurality of paper ejection trays (bins) and performs gathering for each set of copies mechanically. The sorter 34 also includes a staple function and a punch function.
Toner that remains on the photoconductor drum 9 is removed by the cleaner 35. Then, the electricity removal lamp 36 radiates light to the photoconductor drum 9 so that the charges on the photoconductor drum disappear.
Given such a sequence of operations in the image scanning part and the printing part, the primary scanning cycle in the image scanning part is equal to the primary scanning cycle of the printing part. Therefore, copying with contraction or enlargement in the secondary scanning direction can be achieved by changing the secondary scanning speed of the printing part or the image scanning part. Typically, copying with contraction or enlargement in the secondary scanning direction is performed by allowing the secondary scanning speed of the printing part to be constant and the secondary scanning speed of the image scanning part to be different from that of copying in equal size. The relationship between the secondary scanning speed V.sub.0 of the image scanning part for equal size copying and the secondary scanning speed V.sub.Z of the image scanning part for copying at a zoom ratio R is as follows: EQU V.sub.Z =V.sub.0 /R.
This conventional method for contraction and enlargement copying is disclosed in Japanese Laid-Open Patent Publication No. 59-63868.
The minimum zoom ratio in the current digital copier is generally 33% or 25%. In the structure as described above, for example, in the case of copying at a contraction ratio of 33%, the image scanning part is required to scan in the secondary direction at three times the speed for equal size copying. In this case, it is necessary to design the torque of the motor and the secondary scanning mechanism so that the image scanning part can operate at a secondary scanning speed of three times the regular speed, in order to copy at the minimum zoom ratio, which is not frequently used. This is one factor that raises the cost.
Furthermore, in the case of a high-speed digital copier that can perform equal size copying at high speed, copying at a zoom ratio of 33% or 25% requires the image scanner to perform secondary scanning at excessively high speed, which is structurally difficult to realize. Therefore, the minimum contraction ratio is restricted to 70% or 50%.
In the case of enlargement copying, for example, of 400%, the image scanning part is required to perform secondary scanning at a speed of 1/4 of that for equal size copying. The secondary scanning speed becomes slower than that for equal size copying, so there are no problems such as shortage of the torque of the motor. However, in the case of stepping motor driving, noise and jitter cause problems, and in the case of servomotor driving, shortage of the servo band width causes a problem. Dealing with these problems is another factor that raises the cost.
Another technique for copying with contraction in the secondary scanning direction is to use a page memory. In this technique, contraction in the secondary scanning direction is effected by changing the ratio of thinning lines for image signals read by the image scanner. Furthermore, enlargement in the secondary scanning direction is effected by interpolating lines. The image signals for one page contracted in the secondary scanning direction are stored in the page memory, and then output to the printing part. According to this technique, the secondary scanning speed of the image scanning part for contraction copying is equal to that for equal size copying.
However, the read images are stored in the page memory and then printed, so the first copy time becomes longer. Moreover, the digital thinning or interpolation is performed at an arbitrary ratio, thereby degrading the quality of images. In order to avoid the degradation of the quality of images, interpolation operation between lines is performed. This, however, requires line buffers for several lines, thereby raising the cost.